THE NORMAL STATE ELECTRONIC-STRUCTURE OF CUPRATE SUPERCONDUCTORS - A COVALENT PICTURE

We propose a view of the cuprate superconductors based on strong covalent bonding within the CuO2 planes. This covalent picture was developed from an alternative analysis of some recently reported spectroscopic and transport properties of several high temperature superconductors. In this view, antiferromagnetic superexchange splits the sigma-x2-y2 orbital, and crystal field splitting leaves the pi-xy* orbital as the highest occupied molecular orbital (HOMO). In undoped materials, electrons in this orbital are localized, with a peak approximately 1.7 eV below the unoccupied half of the sigma-x2-y2* band. With doping, this orbital broadens into a band where the Fermi energy overlaps the energy of the lowest unoccupied state in the sigma-x2-y2 band. Further, the axial ligand found on all p-type cuprate superconductors raises the energy of the nearly-localized sigma-z2* orbital to an energy slightly below the top of the pi-xy* band. Excitations of electrons in these three bands yield the combination of properties found in the normal state of the cuprate materials.